Electrophysiological characteristics and diagnostic criteria of chronic inflammatory demyelinating polyneuropathy(CIDP) Masayuki Baba 1 1Department of Neurological Science, Hirosaki University School of Medicine Keyword: 伝導ブロック , conduction block , 時間的分散 , temporal dispersion , 伝導遅延 , conduction delay , 多巣性脱髄 , multifocal demyelination pp.501-511
Published Date 2003/8/10
DOI https://doi.org/10.11477/mf.1431100333
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 CIDP is an idiopathic acquired demyelinating neuropathy characterized by chronic relapsing course. The prevalence rate in Aomori Prefecture of Japan was estimated as 2.2 per 100,000 in 1999. The diagnosis of CIDP from clinical picture alone may be possible, nevertheless, electrophysiological demonstration of disseminated demyelinative foci in the peripheral nerve is essential, since many axonal neuropathies has symptomatologic similarities to CIDP.

 Abnormal amplitude reduction(AAR)of proximally evoked compound muscle action potential(CMAP)is a widely accepted index for diagnosis of acquired demyelination. Since diagnostic criteria of possible CIDP suggesting more than 20%reduction as AAR was published by the American Academy of Neurology(AAN),a word “conduction block” has been widely accepted as a diagnostic term rather than physiological definition. The AAN criteria have inevitably been met criticism if 20%reduction is too small for definite conduction block:some electrophysiologists insist that 40%to 60%reduction should be adopted. On the other hand, 40%of CIDP patients may not meet the AAN criteria. The sensitivity and specificity of AAN criteria may be, thus, not so high, because the criteria is based on a rough CMAP change after only two stimulation points study, wrist and elbow, or ankle and knee. To be convinced of multifocal demyelination, one can apply multiple stimulating points(inching technique)to search small foci showing CMAP change. Careful study with inching technique, conduction block of single motor unit potential or a group of some motor unit potentials could convincingly be demonstrated.

 Conduction block in sensory fibers is more difficult to be convinced, since dispersed or small sensory compound action potential(CSAP)become easily unrecordable by surface electrodes. Near nerve recording is essential for it and often useful to demonstrate dispersed polyphasic CSAP of low amplitude. Somatosensory evoked potential is another powerful tool to reveal multifocal sensory involvement in the extremities.

 Practically, to differentiate conduction block from temporal dispersion is not essential, since these two findings both indicate presence of demyelination or remyelination. When the CMAP changes are really multifocal, diagnosis of acquired demyelination is more convincing. When conduction abnormalities are uniform and generalized, the diagnosis of CIDP should be reconsidered, because uniform change in CMAP is a distinctive feature of inherited neuropathies. Discrepant fall in motor conduction velocity to relatively normal distal latency and the reverse are also frequent findings in CIDP.

 Temporal dispersion by some delayed motor unit potentials can be hidden by majority of normally conducting motor unit potentials. Submaximal stimulation can revealed the occult dispersed units with relatively low threshold. When distal CMAP is dispersed and of low amplitude, proximal AAR may not meet diagnostic criteria of demyelination.

 F-wave block is an odd finding, since disappearance of F-wave or decreases population of F-wave is often found in amyotrophic lateral sclerosis, or other spinal muscular atrophies, too. When F-wave block is an only abnormal finding, we have to wait for additional findings suggesting generalized denervation or demyelination.

 In demyelinating neuropathies, complex a-waves are seen frequently. The size of a-wave in CIDP is sometimes as quite big as a normal motor unit potential. Complex polyphasic a-waves consisted of a few motor units are occasional findings. These features strongly indicate that a-waves are not originated from axon branching, but from cross-talk at demyelinative foci. Some late a-waves may have the fastest conduction, since they sometimes jump to right in front of main CMAP when strong stimulating current is applied.

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